Looks mixed blood, could be a slightly unusual Coastal, definitely not a pure Murray Darling. Incorrect names are very common.
Incidentally, so far there are no snake breeds of any type anywhere in the world. Murray Darling is a subspecies. Breeds are things created by humans (humans create them by breeding them). German Shepherd Dog, Siamese Cat, Jersey Cow, etc, are examples of breeds. This is also different from morphs (eg albino, jaguar, striped, etc).